The present invention relates to the removal of volatile constituents from the product of a polymerization process, and more particularly to an apparatus which utilizes centrifugal forces to produce reduced pressure to remove such volatile constituents from heat sensitive polymers.
Many thermoplastic polymers are prepared by the polymerization of a suitable monomer or mixture of monomers, generally in the presence of a volatile liquid media. Typically, the initial product is produced in a solution, mass, emulsion, or suspension from which it is necessary to isolate and remove unreacted monomer and solvent from the polymer product. The presence of residual monomers has many deleterious effects on a polymer. For example, they lower the strength characteristics of the polymer, produce internal bubbles in the product, may impart undesirable tastes and odors to food products that come into contact with the polymer, and reduce the polymer's resistance to degradation from environmental conditions such as heat and exposure to light. Thus, in the production of many polymers, it is necessary to remove residual unreacted monomer and/or solvent from the polymer during a finishing operation.
Such a finishing operation may involve the application of heat to the product. Heating to the vaporization temperature of the monomer or solvent results in a gradual dissipation of monomer and/or solvent over an extended period. However, long residence times increase the commercial costs of any finishing operation. While increasing the temperature of the product increases the rate of removal of volatile monomers and solvent, such higher temperatures also have an adverse effect on the properties of the polymer product and may result in the generation of further monomer as the polymer degrades. Thus, undesirably high levels of monomer may remain in the product even after extended exposure to high temperatures.
A number of apparatuses and methods have been used previously to remove small amounts of volatile materials from synthetic thermoplastic resins such as polyethylene, polystyrene, and polyvinylidene chloride. Because many of these resins are sensitive to heat, and tend to degrade upon exposure to heat, it is desirable to perform the finishing operation on the polymer in such a manner that the polymer is exposed to elevated temperatures for a minimal amount of time. Thus, conventional devolatilization procedures are often undesirable because of the length of time involved.
Conventional devolatilization systems are typically based on a gravity flow system. That is, the polymer is pressure fed through a heat exchanger into a gravity flow vacuum chamber where the volatiles are stripped from the polymer as it falls through the chamber. The polymer then continues falling under the force of gravity toward an outlet where it is removed from the vacuum chamber. In such a system, normal fluid flow control of the polymer is impossible because of the nature of the system, i.e., a reduced atmosphere with insufficient force to generate thin films for flashing off volatiles. Additionally, the polymer may become hung up on the walls of the vacuum chamber or may become otherwise trapped in stagnant or recirculating pools of product. In either case, control over the residence time of the polymer is lost, and certain portions of the polymer may be in the system for a time sufficient for polymer degradation to occur. This results in a product containing gels and carbon particles rendering it unsuitable for certain end uses.
Attempts to overcome these problems by varying the temperature in the devolatilization system have not proven successful. If lower temperatures are employed, the residence time of the polymer becomes excessively long because of the higher viscosity of the polymer at the lower temperature. Further, the rate of polymer removal from the system is slowed due to the higher product viscosity, and the rate of volatile removal is lowered because of lower vapor pressure of the volatiles.
If higher temperatures are used, undesirable degradation of the polymer occurs which is generally in proportion to the processing temperature and length of time the polymer is exposed to that temperature. Where, the polymer involved is particularly heat sensitive, exposure to higher temperatures often results in severe polymer degradation, the production of black carbon particles, and charred resin. Moreover, while the higher processing temperature speeds volatile removal initially, thermal degradation of the polymer produces additional monomer in the system. In some instances, the monomer regeneration rate is such that the overall concentration of volatiles in the polymer is increased.
Accordingly, there remains a need in this art to provide an efficient system for the removal of volatiles from polymer products which can be accomplished in a sufficiently rapid manner and which does not result in the degradation of the polymer.